The present invention relates to a differential transmission cable capable of suppressing radiation noise and a joint for connecting such a differential transmission cable to a device.
The differential transmission cable is a transmission cable for use in a differential transmission system, which is a digital transmission system for sending digital signals in apposite directions through a pair of lines with or without bias. Seeing the lines of the differential transmission system from far, high-frequency components are theoretically cancelled, emitting no radiation noise. Accordingly, the differential transmission system is put into practical use to send digital signals between computers and their peripheral devices. The transmission standards of digital signals transmitted through differential transmission cables include a universal serial bus (USB), IEEE1394, etc.
Because outside noise conveyed in the air are equivalently induced in a pair of lines, there is theoretically no likelihood that the outside noise disturbs the transmission of signals. Accordingly, it has been considered that the differential transmission cables are free from problems of radiation noise and outside noise.
However, actual differential transmission cables are likely to generate radiation noise. One of the causes therefor is that noise current and thus radiation noise are generated by the uncancelled difference between a pair of digital signals that is caused by the destroyed balance of the differential transmission signals. Another cause is that conduction noise generated from personal computers and peripheral devices flow through the transmission cables, generating radiation noise.
Those harmful among these radiation noises should of course be coped with. One of the conventional means for suppressing noise is disclosed, for instance, by Japanese Patent Laid-Open No. 8-84124. This means is made for usual transmission cables. Specifically, it includes induction elements, capacitance elements, common-mode choke coils, low-pass filters (T-type filters), etc. inserted into transmission systems.
Another cable having noise-suppressing means may be a cable comprising differential transmission lines. Japanese Patent Laid-Open No. 10-208818 discloses a differential transmission cable comprising two sets or more of differential transmission lines, each set of differential transmission lines comprising ferrite beads. In this differential transmission cable, when signals of the same phase are transmitted through a pair of transmission lines, magnetic flux generated by the signals does not adversely affect (interfere with) the other sets of transmission lines.
However, the above conventional differential transmission cable attenuates high-frequency components in digital signals, resulting in the deformation of the digital waveform. On the other hand, if the quality of the digital signal is to be kept, only insufficient attenuation would be obtained in radiation noise. Because the digital signals comprise basic frequency components and harmonic wave components of odd power higher than the basic frequency by one order or more, the removal of the harmonic wave components (high-frequency components) means nothing other than the degradation of the quality of digital signal waveform. That is, keeping the quality of the digital signal waveform is in a contradictory relation with the suppression of radiation noise. The above conventional technology is thus unsatisfactory in suppressing radiation noise while keeping the quality of the digital signal waveform.
Accordingly, an object of the present invention is to provide an inexpensive, highly reliable, differential transmission cable capable of dramatically attenuating radiation noise while fully keeping the quality of digital signal waveform, and a joint for connecting such a differential transmission cable to a device.
Thus, the first differential transmission cable of the present invention comprises a cable having at least one pair of differential transmission lines, one or more transmission transformers, and connectors at both ends, the distance between a center of the transmission transformer and a tip end of the closest connector being within 0.1 m, and the distance between a center of the transmission transformer and a tip end of the farthest connector or between a center of the transmission transformer and a center of the other transmission transformer being 0.5 to 4 m.
The second differential transmission cable of the present invention comprises a cable having at least one pair of differential transmission lines, one or more transmission transformers, and connectors at both ends, the distance between a center of the transmission transformer and a tip end of the closest connector being within xcex/20, and the distance between a center of the transmission transformer and a tip end of the farthest connector or between a center of the transmission transformer and a center of the other transmission transformer being xcex/4 to 2xcex, wherein xcex represents the wavelength of a noise that radiates from the differential transmission cable and should be attenuated.
In one embodiment of the present invention, one transmission transformer is disposed in the vicinity of one end of each pair of differential transmission lines. In another embodiment of the present invention, two transmission transformers are disposed in a pair of differential transmission lines, one transmission transformer being disposed near one end of a pair of the differential transmission lines, and the other transmission transformer being disposed near the other end of a pair of the differential transmission lines.
The transmission transformer desirably has such small shape and size that it can be contained in the differential transmission cable in a compact manner. For this purpose, the transmission transformer is preferably a sintered transformer of an integral laminate type or a thin-film-type transformer. The transmission transformer preferably has a coupling coefficient of 0.65 to 0.98 at 100 MHz.
The transmission transformer is preferably disposed in the differential transmission lines and/or the connector. When the transmission transformer is mounted in the connector, the transmission transformer is mounted onto a substrate, which is then resin-molded in the connector. Also, when the transmission transformer is disposed in a housing of a connector, the housing is preferably provided with lands for mounting the transmission transformer and a holder for positioning the transmission transformer. The holder for positioning the transmission transformer is preferably in a shape having a hook in an upper portion.
A part of each land for mounting the transmission transformer is preferably constituted by terminals of the connector. Also, each land is preferably provided with a waist portion. The lands on the side of mounting the transmission transformer may be of a contact pin type. In the resin-molding of parts, a molten resin is injected into a die in which parts to be assembled are disposed, to provide an integral molding.
The joint for connecting at least one pair of differential transmission cables for transmitting differential transmission signals to a device has a structure in which a transmission transformer is mounted in pairs of transmission lines, the transmission transformer being mounted onto a substrate, which is resin-molded in a male connector.
It is preferable that the substrate is resin-molded in at least one of male and female connectors, and that the joint is as short as 0.1 m or less. It is also preferable that the transmission transformer is disposed in a housing of the male connector, and that the housing is provided with lands for mounting the transmission transformer and a holder for positioning the transmission transformer.
It is further preferable that the transmission transformer is disposed in at least one of male and female connectors, that the housing is provided with lands for mounting the transmission transformer and a holder for positioning the transmission transformer, and that the joint is as short as 0.1 m or less.
A part of each land for mounting the transmission transformer is preferably constituted by terminals of the connector. Also, each land for mounting the transmission transformer preferably has a waist portion. Further, the lands on the side of mounting the transmission transformer are preferably of a contact pin type. Further, the holder for positioning the transmission transformer is preferably in a shape having a hook in an upper portion. Further, noise-suppressing parts other than the transmission transformer are preferably inserted together into the connector.